Low-voltage power switch and arc fault detection unit with compensation due to phase shifting

ABSTRACT

A Rogowski coil is used for determining the magnitude of the electrical current of a conductor of a low-voltage AC circuit, which outputs an analogue voltage which is equivalent to the magnitude of the electrical current of the conductor. The Rogowski coil is connected to an analogue integrator, which is followed by an analogue-digital converter, which converts the integrated analogue voltage into a digital signal which is further processed by a microprocessor in such a way that the phase shift generated by the Rogowski coil and the components connected downstream of the Rogowski coil is compensated such that there are in-phase current values for the detection of error situations in order to protect the low-voltage AC circuit, in particular for a low-voltage power switch or an arc fault detection unit.

PRIORITY STATEMENT

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP2018/076271, which has anInternational filing date of Sep. 27, 2018, which designated the UnitedStates of America, the entire contents of which are hereby incorporatedherein by reference.

FIELD

The present application generally relates to a low-voltage power switchfor a low-voltage AC circuit in accordance with the preamble to patentclaim 1, an arc fault detection unit for a low-voltage AC circuitaccording to the preamble to patent claim 3, and a method for arc faultdetection for a low-voltage AC circuit according to the preamble topatent claim 9.

BACKGROUND

Power switches are protective devices that function similarly to a fuse.Power switches monitor the current flowing through them by means of aconductor, and interrupt the electrical current or the flow of energy toan energy sink or to a consuming unit, a process known as tripping, whenprotection parameters such as current limit values orcurrent-time-voltage limit values, i.e. when a current value is presentfor a certain period of time, are exceeded. The current limit values orthe current-time-voltage limit values that have been set arecorresponding tripping conditions. The interruption is, for example,achieved through contacts of the power switch that are opened.

For low-voltage circuits, installations and grids in particular thereare various types of power switches, depending on the electrical currentlevel provided in the electrical circuit. In the sense of the invention,power switch refers in particular to switches such as are employed inlow-voltage installations for currents, in particular rated currents, ormaximum currents of between 63 and 6300 amperes. In particular, closedpower switches for currents between 63 and 1600 amperes, in particularof between 125 up to 630 or 1200 amperes are employed. Open powerswitches are in particular used for currents between 630 and 6300amperes, in particular of between 1200 up to 6300 amperes.

Open power switches are also known as air circuit breakers, abbreviatedto ACB, while closed power switches are known as molded case circuitbreakers, abbreviated to MCCB, or compact power switches.

Low-voltage refers to voltages up to 1000 volts AC or 1500 volts DC.Low-voltage in particular refers to voltages greater than the lowvoltage having values of 50 volts AC or 120 volts DC.

In the sense of embodiments of the invention, power switch refers inparticular to power switches with an electronic trip unit serving as acontrol unit, also referred to as an electronic trip unit, abbreviatedto ETU.

In low-voltage AC circuits, low-voltage installations or low-voltagegrids, short-circuits are usually accompanied by the occurrence of arcfaults such as parallel or series arc faults. In high-power distributionand switching installations in particular, these arc faults can lead todisastrous damage to operating means, installation parts or entireswitching installations if not switched off quickly enough. In order toavoid a longer-lasting, wide-area failure of the energy supply, and toreduce damage to both persons and generally, it is necessary to detectand extinguish arc faults of this sort, in particular high-current orparallel arc faults, within a few milliseconds. Conventional protectionsystems for energy supply installations cannot offer a reliableprotection that meets the necessary temporal requirements.

Switching arcs such as occur during electrical switching, in particularat the contacts, are not intended here.

Arc faults refer to arcs such as occur in the presence of electricalfaults in the circuit or in the installation. These can, for example, becaused by short-circuits or by poor connections.

If a current flows in a faulty phase conductor, for example with areduced cross-section resulting, for example, from crushing, this, as aresult of the reduced current carrying capacity, leads to anunacceptable degree of heating and, as a result of that, potentially tomelting of the conductor and to a serial arc fault.

If a (near) short-circuit occurs to another phase conductor, this isreferred to as a parallel arc fault.

Parallel arc faults can, for example, result from ageing of theinsulation material or from the presence of conductive contaminationbetween the phase conductors. They can occur between two different phaseconductors, between a phase conductor (L) and the ground conductor (PE),or between phase conductors and the neutral line (N). In many cases, theparallel arc also arises as a result of a serial arc, caused for exampleby unprofessional working or incorrectly dimensioned means of contact.

If an arc has the properties of a parallel and of a serial arc fault, itis referred to as a combined arc fault.

In general, arc faults cause a conductive, faulty connection betweenconductors and installation parts.

Various possibilities for detecting arc faults have in the meantimebecome known. One possibility is that of detecting an arc fault frommeasured voltage and current values by evaluating characteristicproperties. This is often done in that the voltage and current valuesare evaluated, or corresponding calculations are carried out, by meansof a microprocessor. Voltage and current values including accuratephases must be available for many algorithms in order to detect an arcfault reliably or to avoid errors in the detection, so thatinterruptions to the low-voltage circuit, in spite of no arc faultsbeing present, do not result.

The voltages are usually ascertained by means of voltage sensors whichusually operate with accurate phase detection.

Measuring resistors, known as shunts, have until now been used forascertaining the currents. The voltage across a known resistance ismeasured here, and the current ascertained from this. An ascertainmentof the current level with an accurate phase is thereby provided.Measuring resistors have, however, the disadvantage that significantpower losses, proportional to the current level, occur in them. Inhigh-current low-voltage AC grids in particular, these cannot beneglected.

Rogowski coils, on the other hand, are available for ascertaining thecurrent level. These deliver a voltage proportional to the differentialof the current. The current level can be ascertained by integrating thisvoltage. Rogowski coils have the disadvantage that the ascertainedcurrent level is not accurate in phase. This is caused by the strayfield inductance of the winding of the Rogowski coil. Rogowski coilshave, on the other hand, the advantage that they exhibit potentialisolation, the ability to withstand high currents, and low physicalsizes. DE 102010011023 A1 discloses in this context a three-pole orfour-pole low-voltage power switch with Rogowski coils acting as currentsensors. FR 3059783 A1 further discloses a method for the manufacture ofa measuring sensor for a power switch.

SUMMARY

At least one embodiment of the present invention enables the use ofRogowski coils for ascertaining current values with accurate phase, inparticular for the detection of arc faults, in particular for alow-voltage power switch and an arc fault detection unit.

Embodiments are directed to a low-voltage power switch, an arc faultdetection unit, or a method.

An arrangement is proposed according to at least one embodiment of theinvention for a low-voltage AC circuit, in particular a low-voltagepower switch or an arc fault detection unit, comprising:

at least one Rogowski coil for ascertaining the electrical current levelof a conductor, in particular Rogowski coils for all the conductors ofthe low-voltage AC circuit, that outputs an analog voltage (P1) that isequivalent to the electrical current level of the conductor.

An analog method is further claimed according to at least one embodimentof the invention, in which the voltage values of at least one conductorof the low-voltage AC circuit are ascertained, in which current valuesof the at least one conductor of the low-voltage AC circuit areascertained with a Rogowski coil, wherein the Rogowski coil outputs ananalog voltage that is equivalent to the electrical current level of theconductor, the voltage values and current values are fed to amicroprocessor that is configured in such a way that an arc faultrecognition is carried out and an arc fault recognition signal is outputwhen at least one limit value is exceeded.

BRIEF DESCRIPTION OF THE DRAWINGS

The properties, features and advantages of this invention described, aswell as the manner in which this is achieved, will be understood withgreater clarity and meaning in connection with the following descriptionof the exemplary embodiments, which are explained in more detail inassociation with the drawings.

In the associated drawings:

FIG. 1 shows a block diagram of a low-voltage power switch;

FIG. 2 shows a block diagram of an arc fault detection unit;

FIG. 3 shows an arrangement according to an embodiment of the invention;

FIG. 4 shows an equivalent circuit diagram of a Rogowski coil;

FIG. 5 shows a first diagram with a voltage curve and a current curve;

FIG. 6 shows an equivalent circuit diagram of a Rogowski coil and anintegrator;

FIG. 7 shows a second diagram with a voltage curve and a current curve;

FIG. 8 shows an equivalent circuit diagram with summarized components;

FIG. 9 shows a third diagram with a first and a second current curve.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

An arrangement is proposed according to at least one embodiment of theinvention for a low-voltage AC circuit, in particular a low-voltagepower switch or an arc fault detection unit, comprising:

at least one Rogowski coil for ascertaining the electrical current levelof a conductor, in particular Rogowski coils for all the conductors ofthe low-voltage AC circuit, that outputs an analog voltage (P1) that isequivalent to the electrical current level of the conductor.

In an embodiment, the following are provided for a low-voltage powerswitch:

an interrupter unit with contacts for interrupting the low-voltage ACcircuit,

an electronic trip unit with a microprocessor connected to the at leastone Rogowski coil and the interrupter unit, that is configured toinitiate an interruption of the low-voltage AC circuit when currentlimit values and/or current-time-voltage limit values of the phaseconductor are exceeded.

In an embodiment, the following are provided for an arc fault detectionunit:

at least one voltage sensor ( ) for ascertaining the level of thevoltage of the conductor (11), in particular voltage sensors for eachconductor of the low-voltage AC circuit,

with a microprocessor that carries out an arc fault detection using theascertained current level and voltage level of the low-voltage ACcircuit, and which outputs an arc fault detection signal when at leastone limit value is exceeded.

The arrangement is designed, according to at least one embodiment of theinvention, in such a way that the Rogowski coil (or each Rogowski coil)is/are (each) connected to an analog integrator, (each of) which is/arefollowed by an analog-digital converter (each of) which converts theintegrated analog voltage (in each case) into a digital signal that is(are) processed by the microprocessor in such a way that the phase shiftgenerated by the Rogowski coil and the components that follow theRogowski coil, in particular the analog integrator, is compensated for,so that current values having accurate phase are ascertained that can beused for the recognition of fault situations to protect the low-voltageAC circuit, and in particular for fault arc recognition.

This has the particular advantage that Rogowski coils can be employed,with said advantages, in particular for the recognition of arc faults,wherein, as a result of the compensation of the phase shift, simplealgorithms can also be used for fault arc recognition, or the algorithmshere deliver reliable results, and incorrect interruptions of thelow-voltage electrical circuit that is to be protected are avoided.

Advantageous embodiments of the invention are disclosed in the claims.

In one advantageous embodiment of the invention, an amplifier isprovided between the Rogowski coil and the analog-digital converter. Adifferential amplifier can in particular be provided here, in order toprovide an increase to the amplitude.

This has the particular advantage that, in addition to ascertainingcurrent values with accurate phase, an adaptation in terms of amplitude,or the accuracy of the amplitude, can take place.

In one advantageous embodiment of the invention, the Rogowski coil, orthe integrator, is followed by a filter, or a filter precedes theanalog-digital converter.

This has the particular advantage that a filtering of the analog signalcan take place in order to mask out interfering components, or toachieve a better frequency limitation of the signal that is to beconverted from analog to digital.

In one advantageous embodiment of the invention, the Rogowski coil ischaracterized by a mutual inductance, a stray field inductance, and awinding resistance;

the analog integrator comprises at least one resistor/capacitorarrangement with a resultant capacitance and a resultant resistance.

This has the particular advantage that, in addition to the selection ofthe important variables of the Rogowski coil, a particularly simplerealization of an analog integrator is available.

In one advantageous embodiment of the invention, current values with thecorrect phase are calculated from the digital signal by means of themutual inductance, the stray field inductance, the resultant capacitanceand the total of the winding resistance and the resultant resistance.

This has the particular advantage that an ascertainment of correct-phasecurrent values is made possible with a few significant values, forexample at least two values.

In one advantageous embodiment of the invention, the correct-phasedigital current values of the current level of the conductor arecalculated by means of the following formula:

${i_{P}(t)} = {{\int{\frac{{LC\frac{d^{2}{u_{C}(t)}}{d^{2}t}} + {RC\frac{d{u_{C}(t)}}{dt}} + {u_{C}(t)}}{M}dt}} + {C\frac{d{u_{C}(t)}}{dt}}}$

This has the particular advantage that a specific calculation option forthe accurate-phase or correct-phase current values is provided.

An analog method is further claimed according to at least one embodimentof the invention,

in which the voltage values of at least one conductor of the low-voltageAC circuit are ascertained,

in which current values of the at least one conductor of the low-voltageAC circuit are ascertained with a Rogowski coil, wherein the Rogowskicoil outputs an analog voltage that is equivalent to the electricalcurrent level of the conductor,

the voltage values and current values are fed to a microprocessor thatis configured in such a way that an arc fault recognition is carried outand an arc fault recognition signal is output when at least one limitvalue is exceeded.

According to an embodiment of the invention, the analog voltage isintegrated and is processed in such a way that the phase shift generatedby the Rogowski coil and the integrator, and possibly by othercomponents, is compensated for in such a way that the correct-phasecurrent values for the fault arc recognition are ascertained and used.

All the embodiments, both in independent and dependent form referred toin the claims, as well as referred merely to individual features orcombinations of features of patent claims, bring about an improvement inthe use of a Rogowski coil, in particular for the ascertainment of faultarcs.

FIG. 1 shows a schematic block diagram of a low-voltage power switch LS.FIG. 1 shows an electrical conductor L1, L2, L3, N of a low-voltagecircuit, for example of a three-phase AC circuit, wherein the firstconductor L1 forms the first phase with the first phase currenti_(p)(t), the second conductor L2 forms the second phase with the secondphase current, the third conductor L3 forms the third phase with thethird phase current, and the fourth conductor forms the neutralconductor N with the neutral conductor current of the three-phase ACcircuit.

In the example according to FIG. 1, the first conductor L1 is connectedto an energy converter EW (for example as part of a converter set) insuch a way that at least part of the current, i.e. a partial conductorcurrent, or the total current of the first conductor L1, flows throughthe primary side of the energy converter EW. Usually, one conductor,which in the example is the first conductor L1, forms the primary sideof the energy converter EW. The energy converter EW is usually atransformer with a core, for example an iron-cored converter. In oneembodiment, an energy converter EW can be provided in every phase, or inevery conductor of the electrical circuit. The secondary side of theenergy converter EW, or of each energy converter provided, is connectedto a power supply unit NT (or a plurality of power supply units), whichprovides a supply of energy, for example a self-sufficient supply, forexample in the form of a supply voltage, for the units of thelow-voltage power switch, in particular for an electronic trip unit ETU,represented by a connection, drawn dashed, of operating voltageconductors BS. The power supply unit NT can also be connected to atleast one or to all the current units SE1, SE2, SE3, SEN, to supplyenergy to the current units—if necessary.

Each current unit SE1, SE2, SE3, SEN is connected to a Rogowski coilRS1, RS2, RS3, RSN to ascertain the level of the electric current of theconductor of the electrical circuit assigned to it. In the example, thefirst current unit SE1 is assigned to the first conductor L1, i.e. thefirst phase; the second current unit SE2 is assigned to the secondconductor L2, i.e. to the second phase; the third current unit SE3 isassigned to the third conductor L3, i.e. to the third phase; and thefourth current unit SEN is assigned to the (fourth conductor) neutralline N.

The Rogowski coils RS1, RS2, RS3, RSN supply an analog voltage A1, A2,A3, AN at their output that is proportional to the level of theconductor current. These are supplied to the first through fourthcurrent units SE1, SE2, SE3, SEN. In the example, the first throughfourth current units SE1, SE2, SE3, SEN are a part of the electronictrip unit ETU. They can, however, also be provided as separate units(s).

The current units SE1, SE2, SE3, SEN serve to process the voltage of therespective Rogowski coils. The current units SE1, SE2, SE3, SEN forexample supply a digital signal P1, P2, P3, NS to a microprocessor MPthat is, for example, provided in the electronic trigger unit ETU.

The transmitted digital signals P1, P2, P3, NS are compared with currentlimit values and/or current-time-voltage limit values that form trippingconditions in the electronic trip unit ETU. An interruption of theelectrical circuit is initiated when these are exceeded. An overcurrentand/or short-circuit protection is hereby realized. This can, forexample, be done in such a way that an interrupter unit UE is providedwhich is connected on the one hand to the electronic trip unit ETU andon the other hand has contacts K for interrupting the conductors L1, L2,L3, N or further conductors. In this case, the interrupter unit UEreceives an interruption signal for opening the contacts K.

FIG. 2 shows an arrangement for fault arc detection with a fault arcdetection unit SLE, in which the same units are provided as in FIG. 1.This comprises at least one Rogowski coil RS1 for ascertaining theelectrical current level of a conductor L1 of the low-voltage ACcircuit, and output an analog voltage A1 that is equivalent to theelectrical current level of the conductor L1. The Rogowski coil RS1 isconnected to a current unit SE1.

It further comprises at least one voltage sensor SS for ascertaining thelevel of the voltage of the conductor L1 or of the conductor of thelow-voltage AC circuit.

It further comprises a microprocessor MP that is connected to thecurrent unit SE1 and the voltage sensor SS, wherein the ascertainedcurrent level and voltage level of the low-voltage AC circuit are usedfor arc fault detection by the microprocessor. If at least one or aplurality of limit values/fault arc limit values is exceeded, a faultarc recognition signal SLES is output. The fault arc recognition cantake place in accordance with known methods, for example a signalprogression analysis of the voltage and/or of the current, adifferential or integrating solution etc.

Only one circuit with two conductors is shown in the example accordingto FIG. 2. A three-phase AC current with or without a neutral line canbe provided in a similar way.

FIG. 3 shows a design of a current unit SE1 according to FIG. 2 orFIG. 1. This comprises an analog integrator INT, connected to theRogowski coil RS1. The analog voltage A1 of the Rogowski coil issupplied to this. Analog integrator refers here to an integrator thatcarries out an integration using discrete components such as capacitors,inductors, resistors etc. in accordance with analog circuit technology.An analog signal is, in other words, integrated.

An integration using digitized signals and digital circuit technology,for example using a microprocessor, is not intended.

The analog integrator INT supplies an integrated analog voltageu_(c)(t). In one variant this is subjected directly to analog-digitalconversion, for example using an analog-digital converter ADU thatoutputs a digital signal P1 to the microprocessor MP.

In various embodiments according to the invention, a filter FI and/or anamplifier V can be provided in any desired sequence between the analogintegrator INT and the analog-digital converter ADU, for exampleaccording to FIG. 3. Alternatively, a filter can also be provided beforethe integrator INT.

The microprocessor MP is designed in such a way that the phase shiftgenerated by the Rogowski coil RS1 and by the components that follow theRogowski coil, in particular the integrator INT, possibly the filter FIand/or the amplifier V, is compensated for, so that correct-phasecurrent values ip(t) are ascertained and used for the arc faultrecognition.

The current unit SE1, or the arrangement/configuration according to FIG.3, can be a part of the low-voltage power switch according to FIG. 1.

FIG. 4 shows an equivalent circuit diagram of a Rogowski coil RS1. Acurrent i_(p)(t) of the associated conductor hereby brings about ananalog voltage u_(r)(t) of the Rogowski coil, which corresponds to theanalog voltage A1. A mutual inductance M is entered on the equivalentcircuit diagram between the two terminals of the Rogowski coil; thiscorresponds to the magnetic coupling. On the basis of this mutualinductance M, a voltage u_(r)(t) or A1 that is proportional to thechanging current in the conductor is output at the output of theRogowski coil.

$u_{M} = {M_{R}\frac{di_{M}}{dt}}$

The stray field inductance L, positioned in series in the equivalentcircuit diagram according to FIG. 4, results in a phase shift of theoutput voltage. The winding resistance RR, positioned in series, resultsin a change in the amplitude. A winding capacitance CR is furthermorepresent, but can be neglected for a practical consideration as a resultof its low magnitude.

As a result of the reactance and resistance located in series accordingto the simplified equivalent circuit diagram of the Rogowski coil, aphase shift results between the primary current in the conductor and theoutput voltage u_(r)(t) or A1 at the Rogowski coil.

FIG. 5 shows a curve that exemplifies this. FIG. 5 shows a first diagramwith a voltage curve u_(r) or u_(r) (t), scaled in millivolts, of theoutput voltage of the Rogowski coil, and a current curve i_(p) ori_(p)(t), drawn in amperes, of the current of the conductor against timet in milliseconds, which is plotted on the horizontal axis.

The phase shift between the current and voltage can be seen clearly.

FIG. 6 shows an arrangement according to FIG. 4, with the differencethat an integrator INT is connected at the output of the Rogowski coilRS1, consisting in the example of an RC network, wherein a resultantcapacitance C, for example in the form of one or a plurality ofcapacitors, is connected between the two conductors, and an integratorresistor (R_(inte)) is connected in series with at least one conductoror both conductors.

The integrated analog output voltage u_(c)(t) is output here at theintegrator INT.

FIG. 7 shows a second diagram with a voltage and current curve accordingto FIG. 5, with the difference that instead of the voltage u_(r) (t) orA1 of the Rogowski coil RS1, the integrated analog voltage u_(c) oru_(c)(t) is plotted in millivolts against time t in milliseconds.

The phase shift is partially, although not completely, compensated forby the analog integration, as can be seen in FIG. 7.

This is frequently not sufficient for the recognition of fault arcs.

FIG. 8 shows an equivalent circuit diagram according to FIG. 6 withsummarized components. The winding resistor RR of the Rogowski coil andthe integrator resistance R_(inte) are combined here into a resultantresistance R, wherein the resultant resistance R is the sum of thewinding resistance R_(inte), R=RR+R_(inte).

FIG. 9 shows a third diagram with a first and a second current curveaccording to FIG. 5, with the difference that instead of the integratedanalog voltage u_(c) or u_(c)(t), the calculated current i_(p) ori_(p)(t)−B is plotted against the time t in milliseconds in comparisonwith the measured current i_(p) or i_(p)(t)−G.

As can be seen, as a result of the calculation according to theinvention using an analog-digital converter ADU and a microprocessor MP,possibly making use of a filter FI and/or an amplifier V and taking theminto account in the calculation, the phase shift between the measuredand calculated current has been almost completely compensated for.Correct-phase current values are thus present, and can in particular beused for the fault arc recognition as well as for the recognition ofother fault situations for protection of the low-voltage AC circuit.

In relation to a parallel voltage measurement that exhibits no phasedelay, accurate-phase or correct-phase current values are thus present,wherein these correct-phase voltage and current values canadvantageously be used for fault arc detection or for the detection ofother fault situations for protection of the low-voltage AC circuit.

In particular for the protection of high-current switching anddistribution installations, numerical algorithms can in future also beused for low voltages. The use of, for example, distance protectionalgorithms makes it necessary for there to be no phase shift between thesampled current and voltage measured values. The application ofmeasuring resistors (shunts) is thus suitable for the currentmeasurement. At the outputs these develop, according to Ohm's law, avoltage proportional to the current, and this can be acquired with avoltage measuring device. The current can be determined by means of aback-calculation. The use of magnetic transducers, of Rogowski coils forexample, for current measurement leads to a shift in the phase of themeasured current, but nevertheless has significant advantages in termsof the potential isolation and of the low physical size in comparisonwith shunts.

The output signal has to be integrated to calculate the current, sincethe measured voltage is only proportional to the current change. Twomethods, on the one hand a numerical integration and an analogintegration on the other hand, can be used for this purpose. Numericalintegrations have problems with large changes in current: the resolutionof a downstream analog-digital converter or A/D converter is limited. Alarge change in the current (e.g. on the occurrence of a short-circuit)leads to very high voltage values that an analog-digital converter maynot be able to resolve. This effect is known as clipping, and leads toincorrect results.

An analog integrator circuit is therefore employed according to theinvention, which can consist, for example, of an R-C network. Anequivalent circuit diagram according to FIG. 6 results in connectionwith the Rogowski coil, wherein the winding capacitance of the Rogowskicoil was neglected.

The phase shift is only partially compensated for by the integrator. Thephase difference is illustrated in FIG. 7. As a result of the phasedifference, an error occurs in the calculated result when distanceprotection algorithms are applied. The use of Rogowski coils hastherefore been rejected in the past.

Now, according to an embodiment of the invention, the current in theconductor, or the primary current, is determined throughback-calculating from the transmission characteristic of the Rogowskicoil and of the integrator, with correct phase (and correct amplitude,if necessary). In the presence of further components that influence thephase (and/or influence the amplitude), these are or can also becalculated out according to an embodiment of the invention. A meshequation is set up for this purpose according to the equivalent circuitdiagram according to FIG. 6 or 8, and solved. On this basis, thecorrect-phase and correct-amplitude primary current of the Rogowski coilcan be determined from the voltage of the Rogowski coil or the voltageof the integrator.

Due to the correct-phase and correct-amplitude determination of theprimary current, it is possible with the invention to compensatenumerically for the transmission characteristic of Rogowski coils.Rogowski coils can thus in future find application in protection devicessuch as low-voltage power switches or fault arc detection units for thedetection of challenging fault situations.

The electrical elements can be summarized according to the simplifiedequivalent circuit diagram of the Rogowski coil with integratoraccording to FIG. 6. An equivalent circuit diagram according to FIG. 8thus results.

By solving the mesh equationu _(M)(t)=u _(L)(t)+u _(R)(t)+u _(c)(t)   (1)

the voltage u_(m)(t) across the mutual inductance M can be determined,and thus, following a numerical integration, for example in themicroprocessor MP, the primary current i_(p) can be calculated. Thevoltage u_(c)(t) is the value measured over the capacitance C of theintegrator INT. The secondary-side current i_(LRC)(t) is calculated inthe first step to calculate the individual voltages:

$\begin{matrix}{{i_{LRC}(t)} = {C\frac{d{u_{C}(t)}}{dt}}} & (2)\end{matrix}$

The voltage across the combined resistance of the Rogowski coil and theintegrator can thus be calculated according to the following formula:

$\begin{matrix}{{u_{R}(t)} = {RC\frac{d{u_{C}(t)}}{dt}}} & (3)\end{matrix}$

The voltage over an inductance is generally calculated according to

$\begin{matrix}{{u(t)} = {L\frac{d{i(t)}}{dt}}} & (4)\end{matrix}$

In accordance with the equivalent circuit diagram according to FIG. 8,the current i_(LRC) is used for the calculation of the voltage u_(L)across the inductance.

$\begin{matrix}{{u_{L}(t)} = {L\frac{d{i_{LRC}(t)}}{dt}}} & (5)\end{matrix}$

u_(L) can be calculated directly from the measured voltage u_(c) byinserting equation 2 into equation 5:

$\begin{matrix}{{u_{L}(t)} = {LC\frac{d^{2}{u_{C}(t)}}{d^{2}t}}} & (6)\end{matrix}$

By inserting equations 3 and 5 into equation 1, the voltage u_(M)(t)across the mutual inductance of the Rogowski coil is calculated:

$\begin{matrix}{{u_{M}(t)} = {{LC\frac{d^{2}{u_{C}(t)}}{d^{2}t}} + {RC\frac{d{u_{C}(t)}}{dt}} + {u_{C}(t)}}} & (7)\end{matrix}$

The integration of the current change is used to ascertain the primarycurrent as follows, taking the mutual inductance M and the nodeequations into account:

$\begin{matrix}{{i_{P}(t)} = {{\int{\frac{{LC\frac{d^{2}{u_{C}(t)}}{d^{2}t}} + {RC\frac{d{u_{C}(t)}}{dt}} + {u_{C}(t)}}{M}dt}} + {C\frac{d{u_{C}(t)}}{dt}}}} & (7)\end{matrix}$

If a calculation is applied in accordance with this, a (quasi-)corresponding calculated, correct-phase current value i_(p)(t)−B isfound in comparison to a measured current value i_(p)(t)−G, according toFIG. 9.

In practice, giving consideration to the secondary-side current of theRogowski coil can be omitted for a sufficiently accurate calculation.

If the integrator INT at the Rogowski coil is supplemented by one or aplurality of filters or filter circuits FI, then these elements areadvantageously taken into account in accordance with an equivalentcircuit diagram in the transmission characteristic. On the basis of theapproach to a solution illustrated, with a mesh equation having been setup, the transmission characteristic can be determined and the primarycurrent can thus be calculated.

An embodiment of the invention has the advantage that the classiccurrent converter (large physical size) can be replaced by compact,lightweight Rogowski coils.

Although the invention has been closely illustrated and described indetail through the exemplary embodiment, the invention is not restrictedby the disclosed examples, and other variations can be derived from thisby the expert without leaving the scope of protection of the invention.

The invention claimed is:
 1. A low-voltage power switch for alow-voltage AC circuit, comprising: a Rogowski coil, for ascertaining anelectrical current level of a conductor of the low-voltage AC circuit,to output an analog voltage equivalent to the electrical current levelof the conductor; an interrupter unit including contacts forinterrupting the low-voltage AC circuit; and an electronic trip unitincluding a microprocessor connected to the Rogowski coil and theinterrupter unit, the electronic trip unit configured to initiate aninterruption of the low-voltage AC circuit upon at least one of currentlimit values or current-time-voltage limit values of a phase conductorbeing exceeded, the Rogowski coil being connected to an analogintegrator followed by an analog-digital converter, to convert anintegrated analog voltage into a digital signal, wherein themicroprocessor is configured to process the digital signal to compensatefor a phase shift generated by the Rogowski coil and by components thatfollow the Rogowski coil, so that correct-phase current values arepresent for recognition of a fault situation to protect the low-voltageAC circuit.
 2. The low-voltage power switch of claim 1, wherein therecognition of the fault situation is a fault arc recognitionascertained with the correct-phase current values.
 3. The low-voltagepower switch of claim 1, further comprising: an amplifier between theRogowski coil and the analog-digital converter.
 4. The low-voltage powerswitch of claim 1, further comprising: a filter following the Rogowskicoil or the analog integrator, or preceding the analog-digitalconverter.
 5. The low-voltage power switch of claim 1, wherein theRogowski coil includes a mutual inductance, a stray field inductance anda winding resistance, and wherein the analog integrator includes atleast one resistor/capacitor arrangement with a resultant capacitanceand an integrator resistance.
 6. The low-voltage power switch of claim5, wherein the correct-phase current values are calculated from thedigital signal via the mutual inductance, the stray field inductance,the resultant capacitance and a sum of the winding resistance and theintegrator resistance, forming a resultant resistance.
 7. Thelow-voltage power switch of claim 6, wherein the correct-phase currentvalues are calculated by integrating over time (dt) in accordance with:${{i_{P}(t)} = {{\int{\frac{{LC\frac{d^{2}{u_{C}(t)}}{d^{2}t}} + {{RC}\frac{{du}_{C}(t)}{dt}} + {u_{C}(t)}}{M}dt}} + {C\frac{d{u_{C}(t)}}{dt}}}}.$8. An arc fault detection unit for a low-voltage AC circuit, comprising:at least one Rogowski coil for ascertaining an electrical current levelof a conductor of the low-voltage AC circuit, to output an analogvoltage equivalent to the electrical current level of the conductor; atleast one voltage sensor for ascertaining a level of a voltage of theconductor of the low-voltage AC circuit; and a microprocessor to carryout a fault arc detection with the electrical current level ascertainedand the level of the voltage ascertained and, upon at least one limitvalue being exceeded by at least one of the electrical current levelascertained or the level of the voltage ascertained, to output a faultarc recognition signal, wherein the at least one Rogowski coil isconnected to an analog integrator, followed by an analog-digitalconverter, to convert an integrated analog voltage into a digital signalto be further processed by the microprocessor, and a phase shiftgenerated by the at least one Rogowski coil and by components followingthe at least one Rogowski coil are compensated for by themicroprocessor, to ascertain correct-phase current values for the arcfault recognition.
 9. The arc fault detection unit of claim 8, furthercomprising: an amplifier between the at least one Rogowski coil and theanalog-digital converter.
 10. The arc fault detection unit of claim 8,further comprising: a filter following the at least one Rogowski coil orthe analog integrator, or preceding the analog-digital converter. 11.The arc fault detection unit of claim 8, wherein the at least oneRogowski coil includes a mutual inductance, a stray field inductance anda winding resistance, and wherein the analog integrator includes atleast one resistor/capacitor arrangement with a resultant capacitanceand an integrator resistance.
 12. The arc fault detection unit of claim11, wherein the correct-phase current values are calculated from thedigital signal via the mutual inductance, the stray field inductance,the resultant capacitance and a sum of the winding resistance and theintegrator resistance, forming a resultant resistance.
 13. The arc faultdetection unit of claim 12, wherein the correct-phase current values arecalculated by integrating over time (dt) in accordance with:${i_{P}(t)} = {{\int{\frac{{LC\frac{d^{2}{u_{C}(t)}}{d^{2}t}} + {RC\frac{{du}_{C}(t)}{dt}} + {u_{C}(t)}}{M}dt}} + {C{\frac{d{u_{C}(t)}}{dt}.}}}$14. A method for fault arc recognition for a low-voltage AC circuit,comprising: ascertaining voltage values of at least one conductor of thelow-voltage AC circuit; ascertaining current values of the at least oneconductor of the low-voltage AC circuit with a Rogowski coil, theRogowski coil being configured to output an analog voltage equivalent toan electrical current level of the at least one conductor; supplying thevoltage values ascertained and current values ascertained to amicroprocessor, the microprocessor being configured to carry out an arcfault recognition and, upon at least one limit value being exceeded byat least one of the voltage values ascertained or the current valuesascertained, to output an arc fault recognition signal, the analogvoltage being integrated and processed further such that a phase shiftgenerated by the Rogowski coil and the integrating is compensated for insuch a way that correct-phase current values for the fault arcrecognition are ascertained and used.
 15. The method of claim 14,wherein the analog voltage or integrated analog voltage is filtered oramplified.
 16. The method of claim 15, wherein compensation of the phaseshift is configured to be carried out in the microprocessor, themicroprocessor being configured to calculate the correct-phase currentvalues through numerical back-calculation via mutual inductance, strayfield inductance, a resultant capacitance and a sum of a windingresistance and a resultant resistance.
 17. The method of claim 16,wherein physical magnitudes of further components are taken intoconsideration in the numerical back-calculation.
 18. The method of claim14, wherein compensation of the phase shift is configured to be carriedout in the microprocessor, the microprocessor being configured tocalculate the correct-phase current values through numericalback-calculation via mutual inductance, stray field inductance, aresultant capacitance and a sum of a winding resistance and a resultantresistance.
 19. The method of claim 18, wherein physical magnitudes offurther components are taken into consideration in the numericalback-calculation.